The IL-1 family of cytokines is composed of 11 different ligands, namely, IL-1α (also termed IL-1F1), IL-1β (IL-1F2), IL-1 receptor antagonist (IL-1Ra or IL-1F3), IL-18 (IL-1F4), IL-1F5 to IL-1F10, and IL-1F11 (or IL-33). IL-1α and IL-1β are known to induce pro-inflammatory activities on binding to type I IL-1 receptor (IL-1RI) and recruitment of the common co-receptor IL-1 receptor accessory protein (IL-1RAcP), whereas IL-1Ra acts as a competitive inhibitor of IL-1 binding to IL-1RI, thus exerting anti-inflammatory activity. Numerous studies reported that IL-18 is a pro-inflammatory cytokine that is an inducer of IFN-γ, whereas IL-33 was described as an immunoregulatory cytokine involved in particular in the control of Th2 responses. New members of the IL-1 family, including IL-1F5, IL-1F6, IL-1F8, and IL-1F9, were identified through searches in DNA databases for homologs of IL-1. In humans and mice, all the genes encoding these cytokines map to less than 300 kb of chromosome 2q, where they are flanked by the IL1A, IL1B, and IL1RN genes. IL-1F6, IL-1F8, and IL-1F9 share 21% to 37% amino acid sequence homology with IL-1 and IL-1Ra, whereas IL-1F5 displays 52% amino acid sequence homology with IL-1Ra, suggesting that IL-1F5 might represent an endogenous receptor antagonist.
IL-1F6, IL-1F8, and IL-1F9 bind to IL-1Rrp2, a receptor of the IL-1R family, and use IL-1RAcP as a co-receptor to stimulate intracellular signals similar to those induced by IL-1, whereas IL-1F5 was shown to inhibit IL-1F9-induced NF-κB activation in Jurkat T cells that over-express IL-1Rrp2. Like IL-1β, all these IL-1 homologs lack a leader peptide and cannot be released through the conventional secretory pathway, although studies suggest that release of IL-1Rrp2 agonists may be controlled by mechanisms different from those regulating IL-1β secretion. To acknowledge the specific biologic effects of these cytokines and to recognize that they all bind to the same receptor, it has recently been proposed to amend the nomenclature of IL-1 homologs. Thus, IL-1Rrp2 is now termed IL-36R and its ligands are named IL-36α (IL-1F6), IL-36β (IL-1F8), and IL-36γ (IL-1F9). In addition, IL-1F5, which has been shown to exert receptor antagonist activities, has been renamed IL-36Ra.
Messenger RNAs for IL-36α, IL-36β, and IL-36γ are highly expressed in several tissues, particularly in internal epithelial tissues, which are exposed to pathogens and in skin. Interestingly, expression of IL-36Ra and IL-36α is significantly up-regulated in IL-1β/TNF-α-stimulated human keratinocytes, and IL-36Ra and IL-36γ mRNA are highly increased in lesional psoriasis skin. Moreover, IL-36γ protein production is enhanced in human keratinocytes after TNF-α and IFN-γ stimulation. Elevated IL-36α mRNA and protein expression was reported also in chronic kidney disease.
Transgenic mice overexpressing IL-36α in keratinocytes exhibit inflammatory skin lesions sharing some features with psoriasis. This phenotype was more severe when transgenic mice were crossed with IL-36Ra-deficient mice, supporting a regulatory function of IL-36Ra in vivo. The inflammatory skin condition in keratinocyte-specific IL-36a transgenic is even more similar to human psoriasis if the mice are treated with 12-O-tetradecanoylphorbol 13-acetate, resembling the human disease histologically, molecularly, and in its response to therapeutics. Moreover, human psoriatic lesional skin transplanted onto immunodeficient mice is normalized when the mice are treated with anti-IL-36R antibody, arguing that the IL-36 axis is required to maintain the lesional phenotype in human psoriatic skin. Taken together, these data indicate that IL-36R ligands, including IL-36α, IL-36β, and IL-36γ, exert proinflammatory effects in vitro and in vivo and that IL-36Ra acts as a natural antagonist, thus mimicking the IL-1/IL-1Ra system.
There is therefore evidence that IL-36R ligands are involved in a number of disease conditions, and there is a need for new therapeutic agents targeting this pathway, in particular for use in the treatment of inflammatory diseases.